Fluorescent screen material and method of making



'latented May 13, 1941 FLUORESCENT SCREEN MATERIAL METHOD OF MAKING Bennett S. Ellefson, St. Marys, Pa., assignor to Hygrade Sylvania Corporation, Salem, corporation'of Massachusetts Masa, a

No Drawing. Application February 3, 1938, Serial No. 188,516

11 Claims.

This invention relates to fluorescent materials and more especially to fluorescent screens for cathode ray tubes, television tubes, oscilloscope tubes, fluoroscopes and the like. I

A principal object of the invention is to Provide an improved method of preparing a fluorescent screen material whereby the fluorescent color can be accurately controlled.

Another object is to provide an improved methd of preparing a fluorescent screen material whereby the material may be given any desired fluorescent hue from greenish to yellowish.

Another object isto provide an improved method of preparing a fluorescent screen material having very high fluorescent translation efilclency under electron bombardment.

A further object is to provide a fluorescent I material which does not lose its efficiency or change color materially because of the usual heat treatment or in connection with' the exhaust schedule employed in making a tube with the material on the interior thereof, and does not lose efflciency or change color materially by reason of electron bombardment when the finished tube is in operation.

A featureof the invention relates to a fluorescent material of the zinc silicate type having a special color-control component incorporated therein.

Another feature rel-ates to a fluorescent material'of the zinc silicate type having incorporated therein a quantity of beryllium oxide in a proportion correlated to the fluorescent color desired in the finished materiaL.

A further feature relates to a fluorescent material of the zinc silicate typehaving inc'orporated therein a special color-control component and an activator material.

A further feature relates'to an improved method of increasing the fluorescent emciency-of a fluorescent screen when under electron bombardment, by subjecting the fluorescent material to a special devitrification process.

A still further feature relate's'to the novel organization and succession or correlation of steps whereby an improved fluorescent material is obtained without the necessity of employing dimextreme precision of control vof atmosphere, temperature, time and the like. I

Other features and advantages not specifically plied to the manufacture of a screen for a' television tube of the cathode-ray type, it will be unw derstood that this is done merely for explanatory purposes and not by way of limitation.,. The fluorescent brightness or fluorescent translation efficiency under electron as the fluorescent hue of zinc-cadmium sulfides are very sensitive to variations-in electron bombardment, temperature, and gaseous atmosphere 1 such as may unavoidably be present in a high vacuum cathode-ray tube, or such as may be purposely present in so-called gas-filled cathode ray tubes. On the contrary, zinc silicate screens are relatively insensitive to such variations. While therefore the zinc silicate type of screen is preferred because of its stability, it is very difficult to. control its fluorescent color. Ordinarily a zinc silicate screen exhibits a greenish fluorescence when bombarded by high speed electrons, which while it may be unobjectionable in certain devices such as Oscilloscopes, may be objectionable in such devices as television tubes. While various methods of imparting a yellowish hue to the zinc silicate screen have heretofore been suggested, they have in general required either a difficult quenching process in preparing the screen material, or they have required an extremely precise control of the surrounding ate mosphere, temperature, and heat treatment duration. I have found that it is possible-to produce a fluorescent screen of the zinc silicate type having a hue which may be controlled between greenish and yellowish, in a relatively simple and uniformly reproducible manner, by replacing part of the zinc oxide of-the ortho-silicate with a predetermined amount of beryllium oxide, in

, addition to the conventional activator. The percent of beryllium oxide present will be a certain amount'for the desired yellowish hue, and will be decreased in amount as the desired hue approaches the greenish. A further color control can be brought about by varying the amount of Mn (added presumably as activator) for the purpose of producing a selectively absorbing and/or fluorescent glass. .In place of Mn, Cr, Co, or V may be used. In all cases, the visible efficiency is shifted to the longer waves with an increase cult quenching processes, or processes requiring substances up to about 2%.

enumerated, will be apparent after a considera- While the invention'will be'describedas ap- The preferred method of preparation of the fluorescents material comprises melting together commercially pure oxides, or commercially pure materials which when appropriately heated yield such'oxid'es. The oxides should be present in such proportions as to yield a glass which has as bombardment, as wellberyllium oxide.

its'primary devitrification product a crystal phase in which a portion of the zinc of the phenacite structure of willemite may have been replaced by ortho-silicate is first to mix the raw materials in commercially pure powdered form, these raw materials including silicic acid, zinc carbonate, In order to duplicate results from time to time, the raw materials should be in a fairlystandard and comparable condition in order that the same weights of raw material shall yield the same quantity of the desired ele ments. One characteristic of finely divided powdered materials is their ability to absorb moisture, and the quantity of such absorbed moisture may vary from batch to batch. For this reason, it is preferred to dry all the raw materials to remove such moisture before making and weighing thereof. As typical examples of the proportions of the various materials for screens of different hues, may be mentioned the following:

Molecularpercent Material A 20 BeO 43 ZnO 37 Bio:

B 15 B20 48 ZnO 37 SiO: 9 Eco 4 54 ZnO 37 SiO:

01' the above compositions, composition A" 40 yields a yellowish fluorescence; composition B yields a greenish-yellow fluorescence; composition C yields a yellowish-green fluorescence. Other graduations can be produced between the yellowish and greenish hues by using molecular percents of BeO between 0 and 20 with the cor-,

responding change in ZnO molecular percent to keep the total of these two components approximately 63 to 66 molecular percent. Preferably each of the above compositions has added to it an activator such as MnFz, the activator being present in approximately 0.5% by weight.

The selected composition is then melted in a suitable crucible which is placed in a furnace whereby the ingredients become intimately associated, the melting preferably taking place at a temperature" of between 1400 and 1500 degrees C. The melted material is then'allowed to cool. resulting in a glass which has as its primary devitrification product a crystal phase in whicha portion of the Zn of the phenacite structure of willemite has been replaced by Be. When the glass hassufliciently cooled it ,is'subjec'ted to a devitrifyingi process to increase the fluorescent I intensity. Forjthis purpose the glass is heated for one or two hours in a suitable crucible at a temperature of about 1Q00 degrees'Q- After devitrification and cooling, the material is ground to a'powder andis ready foruse as-a fluorescent material. A particular advantage of the method is, that-it adapts itself in a convenient way to 1 produce a free flowing powder. This is of im-.

'portancefor the application of thefluorescent' material to a support by the so-called air settling process. As is well known, there is a lowerv aceaoer mining experimentally the proper grinding period which furnishes the desired particle size. This is obviously much simpler than the growing oi. large crystals of the desired size from smaller 10 particles. A still better uniformity of the tieslred'particle size can be obtained, if after grinding forv the most favorable period of time, a selection of the most desirable range of particle size is made by elutriation. If the intensity of the fluorescence is to be increased the above described devitrifying process may be repeated until the desired intensity is attained. When used to form a television screen it is deposited on the window portion of the television tube in any well known manner, as for example air flotation settling, liquid settling, or by being mixed with ing operation.

( While various proportions and materials have been described herein, it will be understood that various changes and modifications may be made I '-therein without departing from the spirit and scope of the invention.

What I claim is:

1. The method of making a fluorescent mate rial which includes the step of mixing ZnO, 810a and BeO and an activator, and melting the mix ture to vitrifyit to form a zinc silicate wherein p part of the zinc ions are replaced by beryllium ions, and then devitriiying ata temperature, lower than the melting temperature to increase the fluorescent intensity the S102 constituting from approximately 33%; molar percent to approximately 37 molar percent of the mixture.

2. The method of making a fluorescent. material which includes the steps of mixing 2110 with $102 and BeO and an activator batch in which the ZnO and BeO together con stitute approximately two-thirds by molecular weight, and melting the mixture to form a zincbgggilium silicate, and then reheating to about 3. The method according to claim '2 in which the Eco is present in the batch in to approximately 25% by molecular weight.

4. Themethod of making a fluorescent material which includes the steps of mixing ZnO,

S102 and BeO and an activator with the Beo'con; 5 vstituting up .to 25% of the mixture by molecular,

, percent and the S102 constituting approximately 33%; molar percent to approximately 37"molar percent, melting the mixture at a temperature above 1400 C. to forma beryllium-zinc silicate, cooling the melted silicate, and increasing'the temperature of about 1000 C.

"5.. The method of making a, fluorescent material which includes the steps oi melting a mix- .turecontaining BeO, ZnO,'SiO'z and an activator in molar proportions-such that the BeO and ZnO together compose approximately about twothirds of the mixture by molecular percent, cooling the melted mixture, and then heating it to approximately 1000 C. to increase the fluorescentintensity. i D L 6. 'Them'ethod of making a fluorescent material which includes the steps of melting afmixure containing BeO, 2210, S10: and an acti vator in molar proportions to form a amounts up fluorescent intensity by subsequently firing at a" such that the B420 2,242,061 and ZnO together compose about two-thirds oi the mixture with the BeO molar percentage being between a trace and 20%, and the activator being present in approximately /z%; cooling the melted mixture, and then devitriiying by heat treatment at about 1000" C. w

'7. The method according to claim 5 in which the BeO. is added in suflicient molecular (proportlons to shift the apparent fluorescent hue in the direction of the longer wave lengths, and the hue is further controlled by the addition of manganese to produce a selective absorbing glaze.

8. The method of making a fluorescent screen of the beryllium-zinc silicate type having an activator constituent which screen exhibits a predetermined yellowish hue, including the steps of melting a mixture of BeO, ZnO, S102 and an activator, the Eco being present in the mixture from trace to as high as 20% by molecular weight in accordance with the yellowish or greenish fluorescent hue desired, and then partially devitrli'ying at about 1000 C.

9. The method according toclaim 8 in which the BeO constitutes less than 15% of the mixture. v

10. The method of making a fluorescent material which includes mixing ZnO, SiOz, BeO and a manganese activatorfr'nelting the mixture to produce a fluorescent silicate glass wherein part of the zinc oxide of the ortho-silicate is replaced by a predetermined amount of BeO, allowing the mixture to cool whereby it vitrifies with a primary devitrification product having a crystal phase wherein a portion of thezinc of thephenacite structure of willemite has been replaced by Be, then reheating the vitrified mass to a temperature of about 1000 C. v

11. The method according to claim 10, in which the percent of the manganese activator ls-varied forthe purpose of producing a selectively absorbing glass;

v BENNETT S. ELLEFSON. 

